1. Field of the Invention
The present invention relates to an acrylic premix which can provide an acrylic SMC (sheet molding compound) or BMC (bulk molding compound) which is suited for molding at high temperature and has excellent molding processability, thickening property and storage stability; acrylic artificial marble excellent in appearance and impact resistance; and a method for producing acrylic artificial marble which gives high productivity.
2. Description of the Background
Acrylic artificial marble prepared by compounding inorganic fillers such as aluminum hydroxide and the like into an acrylic resin has various prominent functions and properties such as excellent appearance, soft feeling, weatherability and the like of a molded article and is widely used for counters such as a kitchen counter and the like, washing and dressing stands, waterproof pan, and other architectural uses. These are usually produced by a casting method in which a so-called premix prepared by dispersing inorganic fillers into an acrylic syrup composed of acrylic monomers containing methyl methacrylate and of an acrylic polymer is filled into a mold, and this is cured and polymerized at relatively lower temperature.
For example, Japanese Patent Application Publication (JP-B) No. 4-58423 disclosed artificial marble molded article excellent in heat-resistance and flexibility obtained by a casting method wherein a premix comprising aluminum hydroxide and syrup components composed of a methacrylate, aromatic vinyl compound and specific polyalkylenepolyol di(meth)acrylate is cured for long period of time (3 hours) at a temperature of 80xc2x0 C.
However, since this acrylic syrup has a lower boiling point, the curing temperature has to be lowered, which requires longer molding time leading to lower productivity. For example, when the curing temperature is 90xc2x0 C. or more to shorten molding time, the above-described acrylic syrup bumps and smoothness of the resulting molded article significantly decreases. In a casting method, due to scarce pressure applied, when a molded article having complicated form is to be produced, a premix tends to cause filling failure in a mold and form of a molded article is restricted.
For improving these defects, there has been conventionally effected investigation for producing acrylic artificial marble by heat and press molding at 100xc2x0 C. or more of SMC or BMC obtained by adding a thickening agent to such a resin syrup.
For example, Japanese Patent Application Laid-Open (JP-A) No. 6-298883 discloses an acrylic BMC for artificial marble excellent in low shrinkage property in heat-curing, prepared by compounding a thermoplastic acrylic resin powder which is poorly soluble in an acrylic syrup. Japanese Patent Application Laid-Open (JP-A) No. 6-313019 discloses an acrylic BMC for artificial marble produced by compounding into an acrylic syrup a resin powder prepared by spray-drying a cross-linked polymer obtained by emulsion polymerization, cracking in molding the BMC being prevented and appearance and thickening stability of a molded article made of the BMC being improved.
However, when these resin powders are used as a thickening agent, there is a problem that aging of a premix used for obtaining an acrylic BMC tends to require longer time (about 24 hours) and productivity lowers.
When the acrylic BMC as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 6-298883 and 6-313019 is molded by curing at a high temperature of 100xc2x0 C. or more for short period of time for enhancing productivity, there is a problem that the resulting artificial marble has decreased impact resistance.
When a BMC obtained by adding a thickening agent to a premix disclosed in Japanese Patent Application Publication (JP-B) No. 4-58423 is press-molded with heating at high temperature, there is a problem that the resulting artificial marble has lower deflection temperature under load, therefore, a molded article is deformed when released from a mold and deficiencies such as whitening and deforming tend to occur.
An object of the present invention is to provide an acrylic premix which can provide an acrylic SMC (sheet molding compound) or BMC (bulk molding compound) which is suited for molding at high temperature and has excellent molding processability, thickening property and storage stability; acrylic artificial marble excellent in appearance and impact resistance; and a method for producing acrylic artificial marble which gives high productivity.
The present inventors have intensively investigated for attaining the above-described object, and as a result, have found that excellent effects can be obtained by compounding an acrylic poly-functional monomer having specific structure, completing the present invention.
Namely, the present invention relates to an acrylic premix comprising 10 to 90% by weight of an acrylic resin composition (A) comprising an acrylic mono-functional monomer (a1), an acrylic poly-functional monomer (a2) containing at least one compound represented by any of the following general formulae (I) to (IV), and an acrylic polymer (b); 10 to 90% by weight of an inorganic filler (B); and 0.01 to 10 parts by weight of a curing agent (C) based on 100 parts by weight of the total amount of the acrylic resin composition (A) and the inorganic filler (B); acrylic artificial marble obtained by curing this acrylic premix; and a method for producing acrylic artificial marble comprising curing this acrylic premix with pressing and heating at a temperature from 90xc2x0 C. to 150xc2x0 C. 
(in the formula (I), R1 represents a hydrogen atom or a methyl group, and k represents an integer from 3 to 30.), 
(in the formula (II), R1 represents a hydrogen atom or a methyl group, and each of 1 and m represents an integer from 0 to 25 and 1+m is 2 to 25.), 
(in the formula (III), R1 represents a hydrogen atom or a methyl group, and n represents an integer from 2 to 25.), 
(in the formula (IV), R1 represents a hydrogen atom or a methyl group, R2 to R5 represent an alkylene group having 1 to 10 carbon atoms, and each of p, q and r represents an integer from 0 to 10 and p+q+r is 2 to 25).